Regional aerosol changes modulate the odds of record-breaking heat extremes

Record-breaking heat extremes imply large health risks and can disrupt critical infrastructure, because societies are often adapted only up to previously observed extremes. Understanding how new records evolve is therefore essential. The probability of record-breaking heat events depends on the regional warming rate. This rate is mainly driven by greenhouse gas-induced global warming and has increased in recent decades. The resulting annual probability of record-breaking heat extremes is additionally modified in a nonlinear way by other regional forcing changes, such as aerosols. Because aerosol concentrations have changed substantially in many regions, they can amplify or reduce the annual likelihood of exceeding previous temperature records. 

We first analyze single forcing large ensemble simulations that isolate the effects of aerosols and greenhouse gases. In Europe, decreasing aerosol concentrations have increased the regional warming rate and thereby the probability of record-breaking heat extremes by about 35% today. In contrast, in South Asia, where aerosol concentrations are increasing, we find a dampening of record-breaking probabilities of about 40%. To evaluate the effect of near-future aerosol reductions, we use simulations from the Regional Aerosol Model Intercomparison Project (RAMIP). In RAMIP, aerosol emissions are reduced from SSP3-7.0 to SSP1-2.6 either globally or only in selected regions. This allows us to analyze the regional effects of aerosol reductions as well as their remote responses. In general, aerosol reductions lead to an increased probability of record-breaking heat extremes.

Finally, we examine recent observed record-breaking events and evaluate whether their regional frequency matches the expected record breaking probabilities from model simulations. We expect that changes in aerosol concentrations contribute to changes in the annual record-breaking probability in regions with major aerosol concentration changes in recent decades, such as Europe, North America, East Asia, and South Asia. Overall, these results suggest that changes in aerosol concentrations are important for the present and near-future probability of record-breaking heat extremes.